All cultivated forms of lettuce belong to the highly polymorphic species, Lactuca sativa, which is grown for its edible head and leaves. As a crop, lettuces are grown commercially wherever environmental conditions permit the production of an economically viable yield.
Lactuca sativa is in the Cichoreae tribe of the Asteraceae (Compositae) family. Lettuce is related to chicory, sunflower, aster, scorzonera, dandelion, artichoke and chrysanthemum. Sativa is one of about 300 species in the genus Lactuca. 
Lettuce cultivars are susceptible to a number of pests and diseases such as downy mildew (Bremia lactucae). Every year this disease leads to millions of dollars of lost lettuce crop throughout the world. Downy mildew (Bremia lactucae) is highly destructive on lettuce grown at relatively low temperature and high humidity. Downy mildew is caused by a fungus, Bremia lactucae, which can be one of the following strains: NL1, NL2, NL4, NL5, NL6, NL7, NL10, NL12, NL13, NL14, NL15, NL16, B1:17, B1:18, B1:20, B1:21, B1:22, B1:23, B1:24, B1:25, B1:26, B1:27, B1:28 (Van Ettekoven, K. et al., “Identification and denomination of ‘new’ races of Bremia lactucae,” In: Lebeda, A. and Kristkova, E (eds.), Eucarpia Leafy Vegetables, 1999, Palacky University, Olomouc, Czech Republic, pp. 171-175; Van der Arend et al. “Identification and denomination of “new” races of Bremia lactucae in Europe by IBEB until 2002.” In: Van Hintum, Th et al. (eds.), Eucarpia Leafy Vegetables Conference 2003, Centre for Genetic Resources, Wageningen, The Netherlands, p. 151; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), Van der Arend et al. “Identification and denomination of “new” races of Bremia lactucae in Europe by IBEB until 2002.” In: Van Hintum, Th et al. (eds.), Eucarpia Leafy Vegetables Conference 2003, Centre for Genetic Resources, Wageningen, The Netherlands, p. 151; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants); IBEB press release “New race of Bremia lactucae B1:27 identified and nominated”, May 2010; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seeds and young plants), “New race of Bremia lactucae B1:28 identified and nominated”, March 2011), Ca-I, Ca-IIA, Ca-IIB, Ca-III, Ca-IV (Schettini, T. M., Legg, E. J., Michelmore, R. W., 1991. Insensitivity to metalaxyl in California populations of Bremia lactucae and resistance of California lettuce cultivars to downy mildew. Phytopathology 81(1). p. 64-70), and Ca-V, Ca-VI, Ca-VII, Ca-VIII (Michelmore R. & Ochoa. O. “Breeding Crisphead Lettuce.” In: California Lettuce Research Board, Annual Report 2005-2006, 2006, Salinas, Calif., pp. 55-68).
Downy mildew causes pale, angular, yellow areas bounded by veins on the upper leaf surfaces. Sporulation occurs on the opposite surface of the leaves. The lesions eventually turn brown, and they may enlarge and coalesce. These symptoms typically occur first on the lower leaves of the lettuce, but under ideal conditions may move into the upper leaves of the head. When the fungus progresses to this degree, the head cannot be harvested. Less severe damage requires the removal of more leaves than usual, especially when the lettuce reaches its final destination.
Of the various species of aphids that feed on lettuce, the currant-lettuce aphid (Nasonovia ribisnigri) is the most destructive species because it feeds both on the leaves of the lettuce as well as deep in the heart of the lettuce, making it difficult to control with conventional insecticides. The lettuce aphid feeds by sucking sap from the lettuce leaves. Although direct damage to the lettuce may be limited, its infestation has serious consequences because the presence of aphids makes lettuce unacceptable to consumers.
Although several known lettuce cultivars exhibit resistance against diseases, irrespective of lettuce type, all the lettuce cultivars affected produce a limited number of leaves that generally are of unequal size and diminished quality with respect to color and shape. This is a distinct disadvantage for processing purposes because leaves either need to be sorted based on size or they need to be cut to a smaller, more uniform size. The first option requires additional labor, with not all sizes usable. The second option has the disadvantage that it creates many cut surfaces, which then are subject to wound-induced browning, resulting in a greatly reduced shelf-life.
Although several known lettuce cultivars exhibit resistance against downy mildew, irrespective of lettuce type, many lettuce cultivars affected produce large leaves that, when cut to smaller size pieces generally result in a lot of cut surface resulting in a diminished shelf life with respect to wound-induced discoloration of these cut surfaces.
At present the attractive red color in pre-packed lettuce mixtures is often provided by plant species other than Lactuca sativa. Use is made of radicchio rosso (Cichorium intybus), red cabbage or red-veined spinach or red-veined chard. The disadvantage of using leaves from other vegetables than lettuce is that the different taste of these other vegetables is often experienced as undesirable. Radicchio rosso is for example a Cichorium intybus that has a bitter taste. For taste red-leaved lettuce (Lactuca sativa) is usually preferred over other red-leaved species.
In many parts of the world it is preferred to grow lettuce in plastic tunnels and glasshouses, instead of in the open field, to avoid the adverse effects of cold temperatures and/or precipitation on plant growth, yield and quality.
The red leaf color of lettuce is caused by anthocyanin in the leaves. So, a deeply red colored leaf corresponds with a very high concentration of anthocyanin in that leaf. Anthocyanin is an antioxidant, which contributes to the healthy image of lettuce. Its expression is promoted by UV-light. UV-light is only partly transmitted by glass and plastic. This means that anthocyanin expression of lettuce grown in a glasshouse or plastic tunnel, i.e. indoor production, is less than in the open field, resulting in less red lettuce plants. To compensate for the reduced anthocyanin expression under glass or plastic, indoor lettuce growers prefer to grow more deeply red lettuce varieties.
Lack of UV-light also promotes cell expansion in young leaves during growth, resulting in large, soft-leaved lettuce plants in indoor production. The large, fluffy plant habit and the soft leaf texture is disliked by growers and processors, because it often results in handling damage during harvest. Indoor lettuce growers therefore prefer lettuce varieties that tend to stay small in size.
Although several known lettuce cultivars can be harvested mechanically at young plant, i.e. the babyleaf stage, no pest and disease resistant lettuce cultivars exist that can be grown under glass or plastic, harvested mechanically at mature stage and still provide crisp leaf pieces that are of small, directly edible size and have an attractive deeply red color. Mechanical harvesting saves labor cost and improves labor conditions in comparison with commonly applied hand-harvesting methods.
When lettuce plants reach the bolting stage, an elongated stalk with flowers grows from within the main stem of the lettuce plant. Once lettuce plants have reached the bolting stage, the leaves produce a bitter taste that is not desirable by consumers. Bolting is thus a very undesirable trait in the production of leafy vegetables such as lettuce. Therefore, slow bolting varieties are very much desired by growers.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.